The invention relates to a liquid crystal device and to a liquid crystal display incorporating such a device. Such a display may comprise a reflective display for use in low power equipment or in bright environments, such as outdoor sunshine, where a back lit display would be difficult to read.
A known type of reflective liquid crystal display comprises a liquid crystal layer disposed between a linear polarizer and a reflector. The liquid crystal layer is switchable between a first state, in which polarized light from the polarizer is incident on the reflector with linear polarization, and a second state, in which light from the polarizer is incident with circular polarization on the reflector. When linearly polarized light is incident on the reflector, it is reflected without effectively changing its polarization azimuth. The display or picture element (pixel) of the display in this mode is reflective and appears bright. In the second mode, the circularly polarized light is reflected by the reflector with the opposite sense of circular polarization. In this case, the display or the pixel appears dark. The liquid crystal layer may provide linear polarization in the absence of an applied electric field across the layer, in which case the display is of the normally white type. Alternatively, the liquid crystal layer may provide circular polarization in the absence of an applied field, in which case the display is of the normally black type.
A known display of this type has a twisted nematic liquid crystal layer. Beynon et al, Proceeding of the International Display Research Conference, 1997, L-34 relates to this type of display and derives values for the liquid crystal parameters of retardation, twist and alignment orientation necessary to produce circular polarization by the layer. Most of these sets of parameters are such that the display brightness approaches a maximum value asymptotically as the electric field across the liquid crystal layer approaches infinity. Thus, the maximum brightness white state cannot be achieved for a finite applied field.
U.S. Pat. No. 5,490,003 and Yang, Euro Display 1996, pp 449 disclose an arrangement where the maximum brightness can be achieved. However, this arrangement has a somewhat chromatic dark state and the maximally bright state occurs at a relatively high voltage.
Untwisted liquid crystal displays of this type exhibit the same problem of being unable to achieve the maximum brightness white state for finite applied field. U.S. Pat. No. 4,767,190 discloses an arrangement of this type in which an additional fixed retarder of relatively low retardation is disposed optically adjacent the untwisted liquid crystal layer with the optic axes of the retarder and the layer crossed. The retardation of the additional retarder is therefore effectively subtracted from the retardation provided by the liquid crystal layer and this allows the maximum brightness white state to be achieved for a finite applied field. However, this technique is not applicable to twisted liquid crystal displays because the direction of the director, and hence the optic axis, varies azimuthally so that perfect cancellation of retardation cannot be achieved.
In order to achieve a high contrast ratio (the ratio of light reflected by the display in its maximal reflection state to light reflected by the display in its darkest state), the dark state should be made as achromatic as possible. This requires that the light incident on the reflector for the dark state should be as close to perfectly circularly polarized as possible for as much of the visible spectrum as possible. In order to improve the achromaticity of the dark state, it is known to provide extra retarders which are optically in series with the twisted liquid crystal layer, generally between the layer and the linear polarizer. For instance, it is known to provide an achromatic quarter wave retarder between the polarizer and the twisted liquid crystal layer as disclosed in Wu et al, Appl. Phys. Lett., 1996, 68, pp 1455.
In the display or pixel dark state, the liquid crystal layer should have minimal effect on the polarization of light from the retarder so that the achromaticity of the display should be determined by the achromatic quarter wave retarder. In the bright state, the liquid crystal layer converts the circular polarization produced by the quarter wave retarder to linear polarization. However, the liquid crystal layer only has zero retardation at infinite voltage so that there is some residual retardation which degrades the achromaticity of the dark state and requires as large a voltage as possible to achieve the best possible dark state. Further, most achromatic quarter wave retarders are formed from combinations of retarders and are therefore usually circular polarizers rather than quarter waveplates, which compounds the problem further.
In the case of twisted liquid crystal layers, the only exception to the problem of residual retardation is in the case where the twist of the liquid crystal is 90xc2x0. In this case, the surface portions of the liquid crystal layer approximately cancel each other when the voltage and hence applied field across the layer exceeds a predetermined value (assuming equal pretilts at both liquid crystal layer surfaces). For instance, Wu et al, Appl. Phys. Lett., 1996, 68, pp 1455, discloses a twisted nematic liquid crystal display of this type including an achromatic retarder. However, such a display cannot achieve as bright a white state as displays in which the liquid crystal twist is different from 90xc2x0 except for much larger retardation.
The term xe2x80x9coptic axisxe2x80x9d as used herein refers exclusively to the slow axis of a birefringent material.
According to a first aspect of the invention, there is provided a liquid crystal device including: a layer of twisted liquid crystal having a first surface director orientation oriented in a first direction at a first surface of the layer and a second surface director orientation oriented in a second direction at a second surface of the layer, the first direction being different from and non-perpendicular to the second direction; a retarder arranged to compensate for a residual retardation of the layer with a first finite applied field across the layer, the retarder having an optic axis which is substantially parallel or perpendicular to the bisectrix of the first and second directions; and a reflector for reflecting light transmitted through the layer.
The retarder may have a retardation substantially equal to the residual retardation and the optic axis of the retarder may be substantially perpendicular to the bisectrix.
The term xe2x80x9cbisectrixxe2x80x9d as used herein is used essentially in its conventional mathematical sense i.e. the direction in the smaller included angle between the first and second directions and equiangularly spaced from each. Further, the term xe2x80x9csurface director orientationxe2x80x9d as used herein is defined as the orientation of the director at an alignment surface projected onto the alignment surface plane of the liquid crystal layer, so that the surface director orientation is the orientation which the director would have in the absence of any surface pretilt. Also, the surface director orientation (SDO) is equivalent to (SDOxc2x1xcfx80). Thus, the first and second directions and t he bisectrix are located in a common plane which is perpendicular to the optical axis of the device.
The retarder may be a waveplate whose retardation differs from xcex/4 by the residual retardation, where xcex is a working wavelength of the device. In one embodiment, the retardation of the retarder may be substantially equal to the sum of xcex/4 and the residual retardation and the optic axis of the retarder may be substantially perpendicular to the bisectrix of the first and second directors. In another embodiment, the retardation of the retarder may be substantially equal to the difference between xcex/4 and the residual retardation and the optic axis of the retarder may be substantially parallel to the bisectrix of the first and second directions.
The direction of twist of the layer is defined to be positive if the liquid crystal director twists clock-wise in the direction of propagation of light through the layer and negative if the direction of twist is anti-clockwise.
The retarder may be disposed between the layer and at least one xcex/2 plate.
The layer may produce a retardation substantially equal to xcex 4 with a second finite applied electric field across the layer, where the second applied field is different from the first applied field and xcex is working wavelength of the device. The second applied field may correspond to zero potential difference across the layer.
The first applied field may correspond to a potential difference across the layer of less than or equal to seven volts.
The layer may be homogeneously aligned.
The layer may be pixellated.
The retarder and the layer may be disposed between a linear polarizer and a reflector. The retarder may be disposed between the layer and a quarter waveplate.
According to a second aspect of the invention, there is provided a liquid crystal display including a device in accordance with the first aspect of the invention.
It is thus possible to provide a device in which a twisted liquid crystal is used as a retarder and the device is capable of providing a retardation substantially equal to zero at a chosen finite voltage. For instance, in the case of single polarizer reflective displays using twisted liquid crystal, the contrast ratio of the display may be increased by a factor of between 2 and 10 compared with known displays of this type. The device can achieve a dark state of good achromaticity for any chosen voltage applied across the liquid crystal layer. For instance, the dark state may be achieved for an applied voltage less than 5 volts so as to allow the device to be used in an active matrix addressing arrangement.
Reflective displays are frequently used in low power equipment because of the lower power consumption. For instance, reflective displays do not require a back light so that the power consumed by a back light is avoided. As the retardation of the retarder increases, the voltage at which the dark state is achieved falls. This allows the driving voltage of a display of this type to be made as low as possible. As a result, it is possible to reduce the power consumption of a display of this type.
The invention will be further described, by way of example, with reference to the accompanying drawings.